This invention relates to thermally-sensitive electrical controls for electric heaters of water boiling containers such as electric kettles, jugs, urns, pans, coffee perculators, laboratory equipment and the like (hereinafter termed containers of the kind referred to) wherein the electric heaters are either immersion heaters or similarly constructed heaters mounted externally of the container in good thermal contact with a container wall thereof e.g. as by being brazed to the underside of the bottom of the container. The heating elements of such electric heaters as aforesaid terminate in so-called cold ends by means of which electrical connections are made to the heater.
Electric immersion heaters (hereinafter termed electric immersion heaters of the first kind described) conventionally comprise a heating element carried by a head which is adapted to be secured in a water-tight manner in or adjacent an opening in the container wall. In use said head mounts externally of said container a thermally-sensitive electrical control (hereinafter termed a thermally-sensitive electrical control of the first kind described) in the form of a switch-on-dry protector switch unit which makes electrical connection with said cold ends and includes thermally-sensitive switch means effective to interrupt the electrical supply to the heater should the element overheat through being switched on dry. Switch-on-dry protector switch units per se are well known and are described in, for example, British patent specification Nos. 1,401,954 and 1,064,643.
Electric immersion heaters (hereinafter termed electric immersion heaters of the second kind described) of the first kind described are also known which additionally carry a tube for egress of vapour or steam from the interior of the container via the head of the heater, the tube being arranged, in use, so as to extend above the water level in the container. In this case the head mounts a thermally-sensitive electrical control (hereinafter termed a thermally-sensitive electrical control of the second kind described) of the first kind described which is additionally effective to interrupt the electrical supply to the heater in response to vapour or steam issuing from said tube upon the water in the container boiling. Such a thermally-sensitive electrical control of the second kind described together with an electric immersion heater of the second kind described is disclosed in British patent specification No. 1,316,436.
In the preferred embodiment of the invention described in specification No. 1,316,436 there is connected to the head firstly a protector switch unit (shown in FIGS. 3 to 5) which incorporates a thermally-sensitive switch for cutting off the power supply to the heating element if the kettle is switched on dry, and also a vapour or steam sensitive switch unit (shown in FIGS. 6 to 8) which incorporates a further thermally-sensitive switch for cutting off the power supply when vapour or steam issues through said tube.
The steam-sensitive switch unit mounts on the protector switch unit and cooperates therewith to provide a line terminal connection to the heater element via the protector switch and the steam-sensitive switch connected in series. The provision of two separate thermally-sensitive switches and their connection in series results in a control which although effective is complicated and hence expensive. Moreover although switch-on-dry protector switch units per se are well known for use in non-automatic kettles (see British patent specification Nos. 1,401,954 and 1,064,643) the construction described in specification No. 1,316,436 necessarily involves substantial modification of a standard protector switch unit thus increasing costs.
More generally it is known from British patent specification No. 1,274,552 to provide a steam or vapour tube or passage either within or along an outer surface of a water boiling container for the flow of vapour or steam from within the container to an outlet in or adjacent the container floor for the purpose of controlling a thermally-sensitive electrical control associated with an electric heater which heater may or may not be integral with the container.
Specifically described in specification No. 1,274,552 is an immersion heater of the first kind described in association with a thermally-sensitive electrical control which as with that described in specification No. 1,316,436 comprises two switch units namely a switch-on-dry protector unit and a steam or vapour sensitive unit mounted one on the other with the attendant disadvantages mentioned above in relation to the control of specification No. 1,316,436.
It is an object of the present invention to provide an improved and simplified thermally-sensitive switch-on-dry and water boiling control.